A conventional XY planar positioning system stacks an X-actuator assembly onto a Y-actuator assembly to form the two orthogonal axes. One significant drawback of this stacking approach is that the positioning accuracy is substantially affected by the errors in each of the individual actuator assemblies. Since the actuator assemblies are coupled together, the errors of an actuator in every direction will all contribute to the overall positioning errors of the system.
Another disadvantage of stacked stages is the poor efficiency of the actuator power used for moving the payload. In such a system an actuator is required to move not only the payload but also the mass of the other actuator stacked on top of it. This necessitates the use of higher capacity actuators to overcome the problem, thereby increasing the cost of the system. Furthermore, the stacking approach reduces the overall rigidity of the system.
Whilst the conventional approach of stacking actuators has intrinsic problems of inaccuracy and low power efficiency, some prior art systems attempt to address the problems of stacking X and Y-actuator assemblies. For example, U.S. Pat. No. 5,648,690 to Hinds reveals a design wherein the actuators are not stacked but are fixed in the same plane in which the payload platform moves. However, this design has the problem of a very poor ratio of platform moving area versus the overall size of the stage footprint. This ratio is referred to herein as the footprint efficiency. Another approach, as disclosed in U.S. Pat. No. 4,676,492 to Shamir, has a similar configuration in that all the actuator assemblies are fixed on the same plane as the moving platform. Instead of using actuators to push and pull the platform at the planes perpendicular to the moving directions of the platform as presented by Hinds, the Shamir configuration realises platform motion with a pair of screw-driven actuators arranged at two sides of the platform in each of the two X-Y directions. The fact that both actuator pairs are located outside of the dimensions of the moving platform still result in a poor footprint efficiency, however.
One approach to resolving the problem of low footprint efficiency is described in U.S. Pat. No. 5,523,941 to Burton et al. In this design, two sets of actuator assemblies are mounted orthogonally below the moving platform with one set positioned on top of the other. However, in order to achieve an improved ratio of moving range versus total footprint as well as suitable platform rigidity, the platform has to be supported by at least four points on the base. As a result, the positioning accuracy of the system is negatively affected. Furthermore, as the vertical distance between the moving platform and the lower actuator assembly is long, the large offset required for the force transmission reduces the stiffness of the system, which is undesirable.